A disk storage device is an indispensable device for storing mass data and programs of a computer system. In case of a failure in a disk storage device, data or a program may be prevented from being read out from or stored into the disk storage device and the entire computer system using the disk storage device may stop. The disk storage device having movable mechanical parts, it is comparatively easy to fail in the computer system. To prepare for failure, it is conventionally known that dual disk storage devices are redundantly constructed so as to have two disk storage devices each of which always stores the same data and programs, and in the event of failure in one disk storage device, the other normal disk storage device replaces the failed device. Such a redundant or backup system is indispensable to applications which need high reliability, for example, financial applications in banks, securities corporations, etc.
For example, Japanese Published Unexamined Patent Applications (PUPAs) No. 61-240320, No. 61-249132, and No. 62-139172 disclose redundant or backup disk controllers having dual construction. Disclosed in the above PUPAs are disk controllers connected between a large central processing unit and disk storage devices, especially designed so that the same data can be stored in each of two disk storage devices to prepare for failure. However, in a large system, the central processing unit is connected to the disk storage devices through the disk controller. In this case, a host interface for connecting the disk controller to the central processing unit is different from a device interface for connecting the disk controller to the disk storage devices. That is, the host interface, called a channel, can transfer data at a high rate, but is not specialized in a certain peripheral unit. On the other hand, since the disk controller controls a certain disk storage device, the device interface between them is specialized in characteristics peculiar to the disk storage device. That is, the prior art does not suggest the construction of an adapter between a host and redundant disk storage devices, in which a host and device interfaces have the same interface design.
In the process of downsizing in new information processing equipment, it is desired to prepare for failure in disk storage devices used for a small computer (personal computer, workstation, etc.) by making redundant construction for them in applications requiring high reliability. In this case, it is advantageous that a system prepared for failure can be built without making alterations to both host computer and disk storage devices by using standard interfaces for the small computer, for the reason that the system prepared for failure can be constructed with a standard computer and disk storage devices which can be easily purchased. Further, if redundant construction is transparent to a host computer, an operating system or application programs on the host computer can handle the redundant or backup disk storage devices prepared for failure the same as in a single disk storage device. This enables an operating system or application programs used hitherto to perform highly reliable information processing. Such an interface is, for example, an SCSI (Small Computer System Interface, ANSI X3.131-1986 by the American National Standards Institute (ANSI)).